1. Field of the Invention
The present invention relates to a method and an apparatus for evaluating the extent of creep damage in high-tension heat resistant steel used in a power plant with precision and in a short period of time.
2. Description of the Related Art
Since thermal power plants in Japan had been constructed intensively for about ten years from 1955 to 1965, over half of the total plants have been operated for more than 100,000 hours. In order to operate the plants safely hereafter, a precise life evaluation is needed for proper maintenance. A destructive test has been hitherto used as the most reliable evaluation method. The problem was that the method consumed much time and cost in comparison to other methods. Recently, a more convenient method, such as a metallographical evaluation method, has been applied instead of the destructive test.
High-tension ferritic steel or austenite stainless steel is used in recent high-pressure steam power plants. The metallographical method has not been applied to these steels because structural change owning to creep damage is small. Hence, evaluation methods for creep damage, such as a method by a convenient hardness measurement, have been proposed.
For instance, the applicant of the present invention disclosed a method for evaluating the rest of the life of ferritic heat resistant steel in Japanese laid open patent publication P1990-248860A. The method comprises the steps of measuring a hardness of both the part influenced by welding heat and a base metal, preparing a working curve chart denoting the relation of creep damage extent of the parts influenced by welding heat with respect to the hardness differences between the parts influenced by welding heat and the base metal, and finding the creep damage extent by applying a hardness difference between the measuring part influenced by welding heat and the base metal to the working curve.
However, a problem of the accuracy of evaluating the creep damage by the above method still remains, because the hardness varies with factors such as the accuracy of the hardness measurement, heat treatment, and aging, besides creep damage.